The Internet Protocol (IP) multimedia subsystem (IMS), defined by the 3rd Generation Partnership Project (3GPP), is an architectural framework for implementing IP-based telephony and multimedia services. IMS defines a set of specifications that enables the convergence of voice, video, data and mobile technology over an all IP-based network infrastructure. The main protocol for IMS is Session Initiation Protocol (SIP). SIP is an application layer control (signaling) protocol for creating, modifying and terminating sessions with one or more participants. Other services, such as Rich Communication Services (RCS) based on GSMA standards, build on IMS to provide additional messaging and media services for mobile subscribers.